The adhesion of platelets is a critical step in the formation of thrombi in blood vessels and on foreign surfaces such as are found in artificial organs. There have been problems of reproducibility, visibility and control of flow in devices used previously to assess platelet adhesion. We have designed and brought to use a flow chamber which allows direct continuous observation of platelet adhesion (and subsequent aggregation) under conditions of controlled flow. The conditions provided by the flow chamber allow simultaneous measurements for different local shear rates, and the use of part of the chamber for study of effects of drugs while the other part of the chamber provides a simultaneous control. We have observed that platelet adhesion can develop in patterns: in particular, platelets adhere in streaks aligned with the local flow. We propose to use the flow chamber to carry out measurements of human platelet adhesion under various flow conditions and coordinate these with the development of quantitation theories of platelet behavior, and measurement of modification by platelet-active drugs.